Blood sample assay method

ABSTRACT

The invention provides an enzymatic method for measuring the concentration of one or more analytes in the plasma portion of a blood derived sample, containing a first and a second component, where said second component interferes with the measurement of said first component. The method includes: i) diluting the sample with a reagent mixture; ii) substantially removing blood cells; iii) using a reagent which serves to temporarily prevent reaction of the second component, to generate a blocked second component; iv) causing the selective reaction of a constituent of each analyte to directly or indirectly generate detectable reaction products, where one of the analytes is the first component; v) monitoring the detectable reaction product or products; vi) relating an amount of the detectable product or products and/or a rate of formation of the detectable product or products to the concentration of each analyte, where the concentration of at least the first component is related to a corresponding detectable reaction product by means of estimating an un-measurable (fictive) endpoint. Step iii) may be carried out at any stage up to and including step iv) but before steps v) or vi). The reagent of step iii) may be applied to the sample separately or may be included in a reagent mixture during steps i) or iv). A corresponding kit is also provided.

FIELD OF THE INVENTION

The present invention relates to the measurement of components such aslipid components in a cell-containing blood sample such as a whole bloodsample. Specifically, the invention relates to the measurement of plasmalipid components such as cholesterol and triglyceride associated withspecific classes of lipoprotein, particularly by means of enzymaticassays. Most particularly, the invention relates to assays such asdiagnostic, prognostic and risk determining assays for use in automatedmethods conducted on a “point-of-care” apparatus.

BACKGROUND TO THE INVENTION

The measurement of components in body samples is a common feature ofclinical assessments. Many diagnoses can be made or confirmed, the stateor progress of a disease elucidated, or the risk of many conditions canbe assessed, from the concentration of a particular analyte in a bodysample, or from the profile of concentrations of several components. Theincreasing number of known correlations between disease states and theconcentrations of one or more analytes makes sample analysis for bothsingle and multiple analytes an increasingly valuable tool. There isthusa correspondingly increasing pressure on clinical laboratories toanalyse ever larger numbers of samples increasingly quickly. To satisfythis, there is a need for assays which are quicker, higher throughput,simpler and/or more completely automated.

There is a growing demand for assays to be carried out at the “point ofcare”. A demand that is increasing with the ongoing transfer of chronicdiseases to primary care. This is beneficial for the patient, whoreceives immediate advice and is left with less uncertainty. It has alsobeen shown that “near patient” testing improves patient compliance,adherence to treatment and therapeutic control (Price. (2001) BMJ332:1285-8). It is also beneficial for the medical practitioner, who canpotentially avoid the need for a multiple appointments and can be morecertain of her judgements.

Point of care assays are exceptionally demanding in terms of the needfor simple manipulation and rapid results. If an assay takes more than afew minutes then much of the advantage of conducting it at thepoint-of-care is lost. Furthermore, although the staff conducting suchassays are likely to be healthcare professionals, they are notanalytical specialists and will not have access to multiple instruments.It is therefore necessary that such assays be designed to rely on theminimum of sample handling. For this reason, and for reasons of time,personnel resources and cost, it is often of great advantage for aplurality of different analyte levels to be determined in a singleoperation using a single sample on a single analytical instrument and asingle test device. This avoids the costly and time consuming-need forextended sample manipulations, multiple test devices or multipleanalytical instruments.

One particular problem with point-of-care methods is that they canseldom accommodate calibration. This is of special concern in enzymebased assay methods since enzymes by their nature are sensitive toinactivation during storage.

A common way to avoid the problem of unknown enzyme activity is todetermine the end-point of the enzyme reaction. The amount ofend-product formed will depend solely on the amount of analyte presentat the start of the reaction, providing that there is sufficient reagentto convert all analyte into product and sufficient time is provided forfull conversion. As long as there is sufficient active enzyme tocatalyse the reaction, the activity of that enzyme will determine onlythe rate of reaction and not the end-point. However, this approachrequires either a great excess of enzyme, which increases cost and/orrequires a sufficiently long assay time, which increases total assaytime. This is of particular concern to point-of-care assays, whichtypically should be completed in 5-10 minutes.

Of special concern are enzymatic reactions which do not produce ameasurable end-point. Such situations include consecutive reactionswhere the next reaction begins before the preceding reaction has reachedan end-point or where parallel reactions occur. Similarly, where onecomponent is reacted in the presence of a blocked second component thatwould react in the absence of the “block” then an end-point can only bereached if the blocking is “permanent”, such as by covalent reaction.However, many blocking reagents have only a temporary effect and thusthe end point of the reaction of a first component cannot be reached ifthe second component becomes to any extent “unblocked” during the periodrequired for that end-point to be reached. Typical examples are themeasurement of lipid components of specific lipoproteins, e.g.cholesterol associated with high-density lipoprotein. Blocking oflipoprotein components is generally a temporary, kinetic, type of block.This makes typical end-point assays difficult or impossible and othermethods such as assays based on fixed-point measurements must be used.These cases are rate-dependent and thus require either fully stablereagents (typically dry reagents), or inclusion of calibrators, to allowcompensation for long-term reagent decay. Dried reagents have thedisadvantage that they are prone to errors upon reconstitution, oftenrequire lengthy reconstitution times, suffer from activity losses duringthe drying process and, in particular in case of in-device driedreagents, add to the cost of production. Calibrators are frequently notcompatible with point-of-care assays.

The structure of a lipoprotein typically has the lipid constituent boundtightly into an inaccessible mass with the protein constituent.Enzymatic reactions will thus generally not affect lipids bound tightlyinto lipoproteins, or will do so at a very slow rate. Reagent mixturesfor the enzymatic reaction of lipids from within lipoproteins thus alsotypically contain reagents, such as surfactants, that help to “unlock”the lipoprotein and expose the lipid constituents to the action of theenzyme(s). “Blocking” of a lipoprotein component is thus often thestabilisation of that component against the action of such surfactantsso that the “blocked” component is not made available for enzymaticreaction. However, over time, the enzymes and/or the surfactants in thereaction mixture will generally begin to cause some degradation of evena blocked lipoprotein component. This then causes a catastrophic effectwhere the action of the reagents begins to open the lipoproteinstructure, which then becomes more reactive and thus more open and soforth. Correspondingly, a “blocked” lipid component can be relied uponto take no measurable part in a reaction for a certain length of time(such as a few hundred seconds) before rapidly and increasingly losingits “block” and causing significant interference. Such temporary blockscannot be used in an end-point reaction because the block will be brokendown before the end point can be reached and thus the result will notrepresent the desired component.

It is also of note that where the component which it is desirable tomeasure contains a relatively small portion of the total amount of aparticular type of lipid (e.g. less than 30% of the total in a typicalhealthy patient) then it is particularly important that when theremainder is “blocked”, the assay is carried out before this blockingcan become significantly undone. This is because a relatively smallunblocking of a large component will have a significant effect upon theresult when the smaller component is analysed.

In view of the above, current point-of-care instruments cannot useliquid reagents because they cannot include a calibrator and cannot useend-point analysis because the endpoint is unreachable or unmeasurable.The only solution that has been available for manufacturers of suchinstruments has thus been the use of stabilised reagents, typically indried form. This however has its own disadvantages. Not only are driedreagents more expensive but their reconstitution is both time consumingand potentially unreliable. Machines using such dried reagents thus tendto have a higher proportion of anomalous results than expected, probablybecause of the occasional failure of the enzymatic reagent to be fullyreconstituted. A method that allowed reactions having no measurableend-point to be used reliably with solution reagents in the absence ofcalibrators would thus be of considerable value.

The most common clinical samples taken for assay are fluids,particularly blood and urine, since these are relatively easy to takeand to manipulate. In blood, it is typically the content of the fluidplasma which is analysed. Some of the most common and clinicallyimportant measurements made on blood-derived samples relate to theplasma lipid contents. The predominant lipids present in blood plasmaare phospholipids (PL), triglycerides (TG), and cholesterol (CH). Ofthese TG and CH are of particular diagnostic interest because of theirassociation with cardiovascular disease, which in turn is one of themost prevalent diseases in the developed world.

Lipids are by their very nature water insoluble and in blood aretransported in complex with apolipoproteins, which render them soluble.These complexes, the lipoproteins are classified into five groups, basedon their size and lipid-to-protein ratio: chylomicrons, very low densitylipoproteins (VLDL), intermediate density lipoproteins (IDL), lowdensity lipoproteins (LDL), and high density lipoproteins (HDL).Chylomicrons are basically droplets of fat, and consist of TG to about90%. Chylomicrons function as vehicles for the transport of dietarylipids from the ileum to adipose tissue and liver and are present in thegeneral circulation for only a short period after a meal.

The four remaining classes of lipoprotein are produced in the liver.Whereas VLDL, IDL, and LDL are responsible for transporting lipids fromthe liver to the tissues, the fifth class, HDL is engaged in the reversetransport of superfluous lipids from peripheral tissues back to theliver for further hepatobiliary secretion. VLDL and IDL have shorthalf-lives and deliver mainly TG to the tissues. LDL and HDL have longerhalf-lives and are the major participants in blood cholesterolhomeostasis. On average LDL and HDL combined carry about 95% of thecholesterol present in blood, with LDL carrying about 70% and HDL about25%.

There are five different types of proteins present in lipoproteins:apolipoprotein (Apo) A, B, C, D, and E, and each type may be furthersubdivided. The apolipoproteins are important for the formation,secretion, and transport of lipoproteins as well as the enzymaticactivities working upon the lipoproteins in the peripheral tissues.Apolipoprotein B (ApoB) is the principal protein in VLDL, IDL, and LDL;in LDL, it is the only protein. HDL is devoid of apoB and its principalprotein is apo-A1.

High concentrations of TG are associated with differentpathophysiological disorders such as diabetes, cardiovascular disease,hyperlipidaemia, hyperglyceridaemia types I and IV, and nephriticsyndromes. Low concentrations are found in hepatic infection andmalnutrition.

From many epidemiological studies it is a well established fact that theCH associated with chylomicrons, VLDL, IDL, and LDL is a major riskfactor for cardiovascular disease (CVD), with increasing concentrationscorrelating with increased risk of CVD. The CH associated with LDLparticles is considered the main risk factor and is by far the largestof these components. CH associated with HDL on the other hand isinversely correlated with risk for cardiovascular disease. The lower theconcentration of HDL, the higher the risk for cardiovascular disease.Therefore it is common practice to determine CH associated with LDLand/or HDL, typically along with total CH, to diagnose and predictcardiovascular disease, as well as in formulating the risk of CVD,potentially in combination with other factors.

Two methods are currently used routinely for quantification of CH. Bothmethods are enzymatic. In the first method is utilised an enzyme chainbeginning with cholesterol esterase and cholesterol oxidase to generatea coloured or fluorescent signal by the generation of hydrogen peroxide.The other method substitutes cholesterol dehydrogenase in place ofcholesterol oxidase and determines the amount of CH in the sample on thebasis of the amount of the produced. NADH or NADPH. These methods relyon the at least partial release of cholesterol from it lipoproteincarrier before the reaction can proceed effectively. Surfactants aregenerally used for this function and are well known in the art.

HDL associated CH may be determined by separating, either physically orby blocking, this class of lipoprotein from non-HDL lipoproteins. Aftermaking the non-HDL unavailable, HDL associated CH is measured using theenzymatic methods of total CH. This reaction cannot, however be run toits end point because the known methods of blocking are temporary andwould become undone before the end-point was reached. The exception tothis is where the non-HDL can be physically separated but this requirestechniques such as centrifugation which are not available topoint-or-care instruments.

Originally, and still much used, was precipitation of non-HDL by usingone of the following:

(i) heparin/Mg2+(Hainlinc A et al (1982) Manual of laboratoryoperations, lipid and lipoprotein analysis, 2nd ed. Bethesda, Md.: USdepartment of Health and Human Services, 1982:151 pp),(ii) phosphotungstate-Mg2+(Lopes-Virella M F et al (1977) Clin Chem23:882-4),(iii) Polyethylene glycol (PEG) (Viikari J, (1976) Scan J Clin LabInvest 35:265-8), and(iv) dextran sulfate-Mg2+(Finley et al (1978) Clin Chem 24:931-3).

The precipitated non-HDL is then removed by centrifugation. The lattermethod is still recommended by the Cholesterol Reference MethodLaboratory Network as reference method for measurement of HDL associatedCH (Kimberly et al (1999) Clin Chem 45:1803-12).

Other methods used separation by electrophoresis (Conlin D et al (1979)25:1965-9) or chromatography (Usui et al (2000) Clin Chem 46:63-72).

The above methods are effective, but require lengthy separation stepsand a number of laboratory instruments. In order to eliminate thelaborious sample pretreatment two different routes have been taken.Point-of-care instruments have been developed that integrate theseparation and quantification of HDL into the test device, which may becassettes or reagent-impregnated strips, such as the Cholestech HDLassay device and method (U.S. Pat. No. 5,213,965).

For the automatic clinical instruments, homogenous methods weredeveloped which did not require a physical separation of non-HDLlipoproteins to measure the HDL associated CH fraction. The non-HDLparticles were blocked by different methods and rendered inaccessible tothe CH metabolizing enzymes. The most recent development has been highlyspecific surfactants that selectively dissolve HDL. IN such a situation,it is the reaction mixture for HDL which effectively “blocks” non-HDL byincluding only surfactants which leave non-HDL lipids in lipoproteinform and thus largely inaccessible to the action of the enzymes.

LDL associated cholesterol is commonly determined computationally usingthe Friedewald equation (Friedewald W T et al (1972) Clin Chem18:499-501):

LDL=Total CH−(HDL+TG/2)

Although convenient and in most cases sufficiently accurate, this methodsuffers from well-known limitations, in particular the need for thepatient to fast before being bled (fasting depletes blood ofchylomicrons) and the requirement for TG levels to be below 4 g/L.Therefore the NIH sponsored National Cholesterol Education Program(NCEP) Adult Treatment Panel III (ATPIII) guidelines have recommendedusing direct measurement of LDL associated CH rather than computing itfrom total CH, HDL associated CH, and TG. However, recent reportsquestion any clinical superiority of directly measured LDL levels overcomputed levels (Mora et al (2009) Clin Chem 55:888-94).

Originally, LDL associated CH was measured using ultracentrifugation(Havel R J et al, J Clin Invest 1955; 34:1345-53). This is still themost used reference method, but evidently requires sample pre-treatment.Homogenous methods were later developed which did not require a physicalseparation of non-LDL lipoproteins to measure the LDL CH fraction (U.S.Pat. Nos. 5,888,827, 5,925,534).

VLDL associated CH was originally measured using ultracentrifugation.This remains a preferred reference method, but during recent yearshomogenous methods for determining VLDL associated CH have beendeveloped. These include methods from U.S. Pat. Nos. 6,986,998 and7,208,287.

CH associated with IDL (also called “VLDL remnants” or “remnant-likeparticles”) is commonly determined using ultracentrifugation, highperformance liquid chromatography or electrophoresis. Two homogenousmethods were recently developed (U.S. Pat. No. 7,272,047 and US2007/0161068) that use specific surfactants to further the selectiveenzymatic decomposition of IDL associated cholesterol.

In recent years several reports have suggested that measurement ofnonHDL may prevent more cardiac events than measurement of LDL (vanDeventer et al Clin Chem (2011) 57:490-501; Sniderman et al (2011) CircCardiovasc Outcomes 4:337-45). In particular nonHDL may be superior atelevated TG levels (Sniderman et al (2010) J Clin Lipidol 4:152-5).NonHDL is currently not directly measured by any known prior assaymethod but computed as the difference between total CH and cholesterolassociated with HDL (nonHDL=Total CH−HDL)”. The present invention,however allows for direct or indirect (calculated) measurement ofnon-HDL components. In one aspect of the present invention, at least oneof the analytes is bound to the group of non-HDL lipoproteins. Inparticular, non-HDL cholesterol is a highly preferred analyte. Non-HDLcholesterol may be measured, for example, in a lipid panel assay alongwith total TG and total CH.

For screening purposes a direct assay for nonHDL will have the advantageof reducing assay time and cost compared to running two assays, Total CHand HDL and computing the nonHDL as the difference (Example 10). Thus,in a further aspect, of the present invention, one analyte will be boundto the non-HD lipoproteins (such as non-HDL cholesterol) and will bemeasured directly (i.e. not by taking the difference between two othermeasurements). This analyte may be measured with or without any otheranalytes. Furthermore, the advantages of measuring non-HDL (eg non-HDLCH) directly extend to assays which used end-point estimation (i.e. thecalculation of a fictive end point as described herein) and also assaysthat use conventional techniques. Thus in a further aspect the inventionprovides an assay for the direct measurement of a lipid component boundto non-HD lipoprotein, such as non-HD cholesterol. A correspondingmethod for assigning a risk of or propensity to CVD is provided bycomparing such a value to an appropriate threshold, such as a thresholdderived from populations of healthy individuals and/or individualssuffering from a high risk or propensity to CVD.

TG is determined routinely in a four step enzymatic reaction, in whichlipoprotein lipase hydrolyzes TG to unesterified glycerol and free fattyacids. The glycerol is then phosphorylated (glycerokinase) and oxidized(glycerol-3-phosphate oxidase) to di-hydroxy-acetone-phosphate andhydrogen peroxide, which is used to generate a coloured, fluorescent orchemiluminescent signal.

As with the measurement of CH of different lipoprotein classes,measurement of TG of specific lipoprotein classes may be performed byseveral methods that exploit different chemical and physicalcharacteristics of the different lipoprotein classes.

As discussed above the measurement of a lipid component of a specificlipoprotein class, e.g. cholesterol in HDL, constitutes a particularproblem for point-of-care assays. Current methods depend on temporarilyblocking the particular lipid component present in lipoproteins otherthan the specific lipoprotein class and then measuring the particularlipid component associated with the specific lipoprotein class. Suchmethods rely on blocking the unwanted lipoproteins with syntheticpolymers and polyanions (U.S. Pat. Nos. 5,773,304, 6,811,994) orantibodies (U.S. Pat. Nos. 4,828,986, 6,162,607) or cyclodextrincombined with polyethylene glycol modified enzymes (U.S. Pat. No.5,691,159), or use specific surfactants (U.S. Pat. Nos. 7,208,287,7,272,047).

Because the blockage is temporary a true end-point for these reactionsis not possible to measure, the only true end-point is the end-point ofthe particular lipid component present in all four classes oflipoprotein. The particular lipid component of the specific lipoproteinis therefore measured either at a fixed time point, chosen so that theparticular lipid component present in lipoproteins other than thespecific lipoprotein does not interfere substantially (usually 5minutes), or kinetically during the first minutes of the reaction. Inboth cases is required either fully stable reagents, i.e. dry reagents,or inclusion of calibrators, to compensate for long-term reagent decay.

A different approach is to eliminate selectively the particular lipidcomponent associated with lipoproteins other than the specificlipoprotein class being analyzed, in an enzymatic reaction not givingdetectable product. The particular lipid component of the specificprotein class is then converted into a detectable product. Several suchmethods have been described making use of surfactants that reactselectively with specific lipoprotein classes (EP 1164376, U.S. Pat.Nos. 5,925,534, 6,194,164, 6,818,414, 6,893,832).

However, in order to accomplish complete elimination the reaction mustbe allowed to reach end-point, and that takes time. The time requiredfor this would be too long time for point-of-care assays and inparticular for point-of-care assays measuring a plurality of analytes,(known as “panel” assays). In addition, these approaches have problemswith inaccuracies caused by an incomplete elimination of the particularlipid constituent in lipoproteins other than the specific lipoproteinbeing analyzed or by non-specific elimination of the particular lipidconstituent in the specific lipoprotein. In practise, then, these typesof assays also require fixed-point measurement and thus rely on eitherfully stable reagents, i.e. dry reagents, or inclusion of calibrators,to compensate for long-term reagent decay.

In view of the above, there is evidently a need for improvedpoint-of-care assays for measuring one or a plurality of plasmacomponents (such as lipid components) and in particular for assaysincluding lipid components of specific lipoprotein classes. It would beadvantageous if such assays allowed for liquid reagents, did not requirecalibrators and/or had short total assay times.

The inventors now have established that it is possible to construct anassay method, suitable for the point-of-care apparatus that usesblood-cell containing samples (such as whole blood), uses liquidreagents, has a short total assay time and does not require calibrators.

The inventors surprisingly have found that it possible to determine anend-point for an enzymatic reaction where that end point istheoretically unreachable and/or in practice unmeasurable. This allowsmany of the advantages of end-point analysis to be applied in situationswhere end-points have previously not been considered. For example, in asequence involving several enzymes, product will be consumed and thusthe end point cannot be reached. Also, in those cases where an end-pointcannot be directly measured, such as in consecutive enzyme reactionswhere the next step starts before the preceding reaction has reached anend-point, or in parallel enzyme reactions where there is a sufficientdifference in the early progress of the reactions.

In situations where the end point cannot be or is not measured, it maybe computed using a suitable algorithm rather than directly measured,with accuracy and CV similar to those achieved with direct measurement.What is needed is for the reaction to be monitored for a sufficientlength of time and then using a suitable algorithm to accurately predictthe end-point. Suitable algorithms and equations have been regularlyused in the art for curve-fitting purposes and are thus well-known, buthave not previously been applied in this way to predict an end pointthat cannot be measured. Typically, the reaction should be monitoreduntil at least 50% of the target analyte has been converted, althoughthis may constitute only a minor fraction of the progress curve.Monitoring the reaction until at least 40%, preferably at least 50% andoptionally at least 60% of the target analyte has been consumed isappropriate in various embodiments.

In one embodiment, it is preferable that the measurement interval ischosen such that any parallel reactions (such as unblocking and reactingof any blocked components) are not significant.

In an alternative embodiment, parallel reactions such as unblocking andreacting of any blocked components may take place to a measureableextent but the influence of such parallel reactions may be to a certainextent corrected for by post-measurement analysis where more than oneanalyte from the sample is measured. For instance when a plurality oflipid analytes are measured, such as in a lipid panel assay, theinterference of nonHDL on HDL measurements (such as by partialunblocking and reaction of the non-HDL component) may be partlycorrected for in an iterative process involving the readings for HDL andtotal cholesterol, according to

Total cholesterol=HDL+nonHDL,

and a predetermined standard curve for the effect of nonHDL on the HDLassay (Example 6).

The inventors also have found that using end-point estimation withreactions that produce measurable end-points may afford substantialadvantages to point-of-care assays using liquid reagents. Measurement ofend-points must take into consideration the effect on assay time of lossof reagent activity with storage time, using estimated end-points avoidsthis and consequently allows for shorter measurement times to be used.

BRIEF SUMMARY OF THE INVENTION

In a first aspect, the present invention thus provides an enzymaticmethod for measuring the concentrations of at least one analyte in theplasma portion of a blood-cell containing sample, wherein the samplecontains a first component and a second component, where said secondcomponent interferes with the measurement of said first component ifsaid second component is present and unblocked, said method comprisingthe steps of

i) contacting said blood cell containing sample with a reagent mixturethat dilutes the sample;ii) substantially removing blood cells to provide a substantially cellfree sample;iii) contacting said sample with at least one reagent which serves totemporarily and/or competitively prevent reaction of said secondcomponent, whereby to generate a blocked second component;iv) contacting said sample with at least one reagent mixture comprisingat least one converting enzyme for at least one constituent of eachanalyte of said at least one analyte, whereby to cause the selectivereaction of the or each analyte to directly or indirectly generatedetectable reaction products, wherein the, or one of, said analytes issaid first component;v) monitoring said detectable reaction product or products;vi) relating an amount of said detectable product or products and/or arate of formation of said detectable product or products to theconcentration of the or each of said at least one analyte in said bloodsample, wherein the concentration of at least said first component isrelated to a corresponding detectable reaction product by means ofestimating an umeasurable (fictive) endpoint;wherein step iii) may be carried out at any stage up to and includingstep iv) but before steps v) to vi) and wherein the reagent of step iii)may be applied to the sample separately or may be included in thereagent mixture of steps i) or iv).

In a preferred aspect, the method will be for the concomitant (e.g.simultaneous) measurement of a plurality of analytes, such as two, threeor more analytes, in said cell-containing sample.

In all aspects of the invention, where detection of a detectable“product” is indicated, this will evidently also allow for the detectionof a detectable reactant or starting material, where context allows. Theonly change that will need to be made in most cases is that the reactantor starting material will be consumed and thus concentration willdecrease. Since product and starting material are typically related byknown stochiometry, decrease in reactant is thus an indirect method fordetecting product. All appropriate product detection may thus be carriedout by means of observing consumption of one or more reactants.

In a further aspect, the present invention additionally provides A kitfor use in determining the concentration of at least three differentanalytes in the plasma portion of a blood-cell containing sample,wherein the sample contains a first component and a second component,where said second component interferes with the measurement of saidfirst component if said second component is present and unblocked, saidkit comprising;

a) a first reagent mixture formulated to dilute said sample;b) a cell separation unit;c) a second reagent which serves to temporarily and/or competitivelyprevent reaction of (block) said second component, whereby to generate ablocked second component;d) at least three further reagent mixtures formulated to cause theselective conversion of said at least three different analytes, wherebyto generate detectable indirect products corresponding to each analyte.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides for an assay method in which theconcentration of at least one analyte in a blood-cell containing sampleis measured.

As used herein, the term “analyte” is used to indicate a component orgroup of components whose measurement is desired. Analytes may thus be asingle component, such as HDL cholesterol, or may be a set ofcomponents. Typically such a set will have a common feature, such as theset of all cholesterol containing lipoproteins.

Among the most preferred analytes for use in the present invention arelipid components, which is to say the total amount of a particular typeof lipid (e.g. TG or CH) in the plasma portion of the sample or thatpart of a particular type of lipid present as a particular lipoprotein(e.g. HDL CH) or a particular set of lipoproteins (e.g. non-HDL CH).

The use of end-point estimation is useful in assays for a singlecomponent, where the end-point of the conversion of a particularconstituent of that component (e.g. conversion of the lipid constituentof HDL) will be estimated by the methods indicated herein. The techniqueis, however, also highly valuable in parallel assays for multipleanalytes (often termed “panel assays”). This applies particularly at thepoint-of-care. In such cases, two, three, four, five or more analytesmay be measured in the assay. Furthermore, the instrument may be able tocalculate the level of other analytes from the measured data and thusmay be able to report on more components than are measured directly (seethe discussions herein for example). In the panel assay embodiment ofthe invention it is preferable that at least three analytes aremeasured.

Where at least two (e.g. at least three) analytes are measured,end-point estimation methods will be used for analysis of one or more ofsuch analytes. Where at least one analyte is a first component whichcannot conveniently/reliably be measured without blocking of anothercomponent then this first component will be analysed by estimation of animmeasurable (fictive) endpoint. In combination with this method,however, other components that could be measured by end-point methodsmay also be measured by end-point estimation in order to reduce the timerequired for the total assay. Thus in one embodiment all analytes aremeasured by end-point estimation methods with at least one analyte beingmeasured by estimation of an unmeasurable (fictive) endpoint.

As indicated above, preferred analytes are lipid components of the bloodsample. Typical analytes will thus be cholesterol, triglycerides orphospholipid, in total and/or associated with a specific lipoproteinfrom the group consisting of VLDL, IDL, LDL and HDL. Cholesterol,triglycerides and/or phospholipid associated with groups of lipoproteins(e.g. non-HDL CH) may also be measured and form further preferredanalytes.

The “first component” referred to herein will typically be a particularlipid associated with a particular lipoprotein (or group oflipoproteins) such as HDL_CH or non-HDL-CH. Thus, typically said firstcomponent is one analyte having a lipid constituent selected fromcholesterol, triglycerides or phospholipid associated with at least onelipoprotein constituent selected from VLDL, IDL, LDL and HDL, such ascholesterol associated with HDL.

The interference caused between the “second component” and themeasurement of the “first component” as indicated herein will generallybe due to the need to distinguish the reaction of the lipid constituentof the first component from that of the second. Typically, therefore,the first and second components will comprise the same lipidconstituent(s). In a typical example, it may be desired to measureHDL-CH (first component and analyte) in the presence of non-HDL-CH(second component). This cannot easily be done without “blocking” of thesecond component because the lipid constituents are both CH and thus ifboth are allowed to react then it is not possible to distinguish betweenthem. In such situations the second component is “blocked” so that onlyor primarily the first component is available for reaction.

There are many methods by which components (especially the “secondcomponent” as discussed herein) may be “blocked”. Many of these methodsare discussed herein and others are known to the skilled worker. If sucha blocking is or can be made permanent (e.g. by some covalent attachmentof a blocking moiety) then end-point estimation is not necessary(although it may still be an advantage in order to speed the assaymethod). However, techniques currently know and/or widely used are“temporary” blocking methods, which is to say the second component isrendered fully or largely uncreative but re-gains this reactivity eitherindependently or in the presence of the enzymatic conversion reagentsand/or surfactants employed in step iv) of the processes describedherein. As used herein, a reagent that serves to “temporarily” preventreaction of a component will prevent significant reaction of thatcomponent under appropriate conditions (e.g. the conditions of step iv))for at least 60 seconds, preferably at least 240 seconds and mostpreferably at least 360 seconds. By significant reaction is indicatedreaction of more than 10%, preferably more than 5% and most preferablyreaction of more than 3% of any constituent (e.g. the lipid part) ofthat component.

Correspondingly, a “blocked” component will not undergo reaction of itslipid constituent to an extent of more than 10%, preferably 5%, morepreferably 3% over a period of at least 60 seconds, preferably at least240 seconds, more preferably at least 360 seconds when subjected toreactive conditions, such as the conditions of step iv) as described inany embodiment herein.

In preferred embodiments, the conditions of steps i), ii) and/or iii)are chosen such that the cells of the blood sample remain intact, orsubstantially intact (e.g. less than 10% of the cells, preferably lessthan 5%, and most preferably less than 3% (e.g. 0.1 to 3%) of the cellssuffer lysis during steps i)-iii). Factors including ionic strength andsurfactant type/concentration are particularly important in thisrespect.

Optionally, reaction accelerators such as disclosed in U.S. Pat. No.6,818,414 may be included in the reagent solution, especially at stepiv) to increase reaction rate and thus decrease assay time.

In the present invention, the method and other aspects relate to themeasurement of at least one analyte. Such analytes will typically belipid analytes and each independently may the total amount of a certainlipid component such as triglyceride, cholesterol, etc or may be a lipidcomponent associated with a specific lipoprotein or a group of specificlipoproteins. Non-exhaustive examples of these include all combinationsof lipid components cholesterol, triglycerides or phospholipid with anyof lipoproteins VLDL, IDL, LDL and HDL, as well as the correspondingparts of each total lipid component not associated with suchlipoproteing. Thus, for example, analytes include HDL cholesterol andnon-HDL cholesterol, the latter being that part of the total cholesterolnot bound to the HDL lipoprotein. Particularly preferred analytesinclude total TG, total CH, nonHDL, LDL, small dense LDL and HDLcholesterol.

In order to measure the parts of certain lipid components associatedwith specific lipoproteins, it is necessary to “block” or renderuncreative either that component or all other lipoproteins having thesame lipid component. This allows measurement of one component in thepresence of another which would normally interfere.

Although this blocking is in itself known and many examples aredescribed herein below, the known methods for achieving this do notresult in a permanent blockage or do not hold the lipid componentpermanently. Rather, after a certain period and/or if the concentrationof the lipid component elsewhere in the sample falls too low then theblockage will begin to fail and the lipid component will becomereactive. Thus, in such situations, a reaction consuming the same lipidconstituent as is constituted in a blocked component cannot be run toits endpoint because the blockage would become ineffective before thatpoint was reached. Such endpoints are described herein as “unreachable”,“unmeasurable” or “fictive” to reflect the fact that they can never infact be reached.

In view of the above, has been assumed in the art that endpoint typeassays cannot be used with blocked lipid components of this type becausethe endpoint is “unmeasurable”. This has resulted in the need to usecalibration or more commonly dried reagents in such assays with all ofthe disadvantages associated with those (as known in the art and asdescribed herein).

However, the present inventors have now established that end-pointestimation techniques can be effectively used even on such unmeasurableendpoints and so that unreachable or unmeasurable (“fictive”) endpointcan still be calculated even if it can never be reached in practice.

With regard to the selective reaction of specific class of lipid orspecific class of lipoprotein, and the conversion of the lipidconstituent of a lipoprotein to detectable secondary analyte, there area number of methods which are well known in the art and any of these aresuitable for use in the present invention. All of the homogeneousmethods described above are suitable and included within the scope ofthe invention. Further details are supplied below and in the referencedcitations.

Two methods are currently used routinely for quantification of CH. Bothmethods are enzymatic and suitable for use in the method of the presentinvention. In the first method, cholesterol esterase convertscholesterol esters into CH. Cholesterol oxidase then converts CH tocholeste-4-ene-3-one and hydrogen peroxide. Finally hydrogen peroxidaseuses the hydrogen peroxide formed to convert 4-aminoantipyrin in thepresence of phenol to generate a coloured quinoneimine compound. Thequinoneimine is monitored by photometry at 500-600 nm wavelength. Otherwell known colour producing substrates (e.g. TMB which product is blueand monitored at 650 nm) or fluorescent or chemiluminescent substratesmay be substituted for the 4-aminoantipyrin/phenol.

The other method substitutes cholesterol dehydrogenase for cholesteroloxidase and determines the amount of CH in the sample on the basis ofthe amount of the produced NADH or NADPH.

With regard to the detection of TG, this is determined routinely in afour step enzymatic reaction, in which lipoprotein lipase hydrolyzes TGto unesterified glycerol and free fatty acids. The glycerol is thenphosphorylated by glycerokinase and oxidized by glycerol-3-phosphateoxidase to di-hydroxy-acetone-phosphate and hydrogen peroxide. In afinal colour forming step hydrogen peroxidase uses the hydrogen peroxideformed to convert 4-aminoantipyrin in the presence of phenol into acoloured quinoneimine. The quinoneimine is monitoredspectrophotometrically at 500 nm wavelength. By selecting theappropriate substrates and phenols the formed coloured products may bemonitored at wavelengths from 450 to 850 nm. Likewise fluorescent orchemiluminescent substrates may be substituted for the 4-aminoantipyrin.Evidently, the final steps in both peroxide-based methods are equivalentand interchangable.

Thus, the enzyme reactions converting the specific plasma lipidcomponent into a detectable chemical product preferably may be performedusing the above described enzyme systems.

For CH:

(i) cholesterol esterase+cholesterol oxidase+peroxidase, or(ii) cholesterol esterase+cholesterol dehydrogenase.

For TG:

(iii) lipase+glycerol kinase+glycerol-3-phosphate oxidase+peroxidase.

As enzymes, commercially available enzymes derived from animals,micro-organisms or plants may be used. Enzymes from specific sources maydisplay selectivity for certain lipoprotein classes, such as lipoproteinlipase and cholesterol esterase from Chromobacterium viscosum orPseudomonas which react preferentially with the lipoprotein class VLDL.Such enzymes may be used to assay for a specific component, either inisolation or in combination with the other selection methods describedherein. The enzymes may be chemically modified so as to change theirspecificity and stability, e.g conjugation of cholesterol oxidase andcholesterol esterase with PEG in order to make the enzymes less reactivewith LDL associated CH (U.S. Pat. No. 5,807,696). Enzymes are typicallyused at concentrations from 100-100,000 U/L.

Measurement of lipid components of specific lipoprotein classes may beperformed by several methods that exploit different chemical andphysical characteristics of the different lipoprotein classes. Ingeneral, these methods rely on the specificity of the enzyme, and/orallow the reaction of the desired component after separating, convertingor rendering inactive those other components which might interfere.Non-ionic, anionic, cationic, and zwitterionic surfactants may beincluded in order to increase the selectivity of the enzymatic reactionsor to increase the rate of reactions. Any suitable surfactant may beused that allows maintaining the intact status of the cells during thecourse of the reactions. One property of non-ionic surfactants ofparticular importance for the status of cells, is the hydrophil-lipophilbalance (HLB). Surfactants with HLB values below 10 and above 13 are inparticular suitable for use in the presence of intact cells. However,surfactants with HLB values between 10 and 13 may also be compatiblewith intact cells depending on the concentrations used, and depending onthe construction and formulation of the assay, such as reaction timesand temperatures used, ion strength, pH and types of salts used in theassay mixture, and the presence of stabilizing substances such as serumalbumin. Examples of suitable surfactants are polyoxyethylene alkylethers (Brij 35 and 78), polyoxyethylene alkyl aryl ethers (Triton X45and X305, Igepal 210 and 272), polyoxyethylene sorbitan monolauratemonolaurate (Tween 80), polyoxyethylene-cooxypropylene block copolymer(Pluronic F68 and L64), telomere B monoether with polyethylene glycol(Zonyl FSN 100), ethylenediamine alkoxalate block copolymer (Tetronic1307), 2,4,7,9-tetramethyl-5-decyne-4,7-diol ethoxylate (Surfonyl 465and 485), polydimethylsiloxane methylethoxylate (Silwet L7600),polyethoxylated oleyl alcohol (Rhodasurf ON-870), polyethoxylated castoroil (Cremophor EL), p-isononylphenoxy-poly(glycidol) (Surfactant 10G),and a polyether sulfonate (Triton X200). Surfactants are used for thispurpose typically at concentrations of 0.001-10%, preferably 0.01 to 1%.

The following methods are among those suitable for selectively reactingthe particular lipoprotein components as described. In all cases, thereferenced published material is incorporated herein by reference.

In homogenous methods to measure the HDL associated CH fraction, thenon-HDL particles have been blocked from reaction by different methodsand rendered inaccessible to the CH metabolizing enzymes. The mostrecent development has been specific surfactants that selectivelydissolve HDL. These include:

(i) In the PEG/cyclodextrin method (U.S. Pat. No. 5,691,159) sulfatedalpha-cyclodextrins in the presence of Mg²⁺ forms soluble complexes withnon-HDL which thereby become refractory to breakdown by PEG modifiedenzymes.(ii) The polyanion method (U.S. Pat. No. 5,773,304) uses a syntheticpolymer together with a polyanion, or a surfactant (US7851174) to blocknon-HDL and make them refractory to solubilization with specificdetergents and enzymatic measurement.(iii) The immunologic method (US6162607) exploits the presence ofapolipoprotein B in all non-HDL and its absence in HDL. Antibodies toapoB block non-HDL for reaction with the cholesterol enzymes.(iv) In the clearance method (US6479249) non-HDL is first consumed in areaction not generating colour. A specific detergent is then addedallowing the enzymes to react with HDL in a reaction generating colour.(v) The accelerator/detergent method (U.S. Pat. No. 6,818,414) degradesunesterified cholesterol of non-HDL using an accelerator to speed up thereaction, and removes the formed H₂O₂ in a process not generatingcolour. In a second step HDL cholesterol is degraded using anHDL-specific detergent in a colour forming process.

Any of these methods, either individually or in combination may beapplied to the measurement of HDL associated CH in the method of thepresent invention. LDL associated CH has been measured by homogenousmethods which did not require a physical separation of non-LDLlipoproteins to measure the LDL CH fraction. Such methods include:

(i) US5888827 describes a method whereby non-LDL is masked bysurfactants and cyclodextrins in the presence of Mg²⁺ and becomesrefractory to breakdown by PEG modified enzymes.(ii) U.S. Pat. No. 5,925,534 describes a method using polyanions andsurfactants to protect LDL in a sample and allow non-LDL to beenzymatically eliminated whereupon addition of a deprotecting reagentallows the enzymatic determination of LDL associated CH.(iii) U.S. Pat. Nos. 6,057,118 and 6,194,164 describe two differentmethods utilizing specific surfactants to selectively eliminate non-LDLassociated CH in an enzymatic reaction before determining LDL associatedCH.

During recent years homogenous methods for determining VLDL associated.CH have been developed. These include:

(i) U.S. Pat. No. 6,986,998 describes a method using albumin andcalixarene to block HDL and LDL, respectively, allowing the selectivedecomposition of VLDL in an enzymatic reaction using VLDL selectiveenzymes from Chromobacterium viscosum.(ii) U.S. Pat. No. 7,208,287 describes a method using specificsurfactants to selectively decompose VLDL in an enzymatic reaction.

Cholesterol associated with IDL (also called “VLDL remnants” or“remnant-like particles”) is commonly determined usingultracentrifugation, high performance liquid chromatography orelectrophoresis. Two homogenous methods were recently developed (U.S.Pat. No. 7,272,047 and US 2007/0161068) that use specific surfactants tofurther the selective enzymatic decomposition of IDL associatedcholesterol. Any of these methods, either individually or in combinationmay be applied to the measurement of LDL associated CH in the method ofthe present invention.

Measurement of TG of specific lipoprotein classes may be performed bymethods that exploit different chemical and physical characteristics ofthe different lipoprotein classes. These methods are thus analogous tothose described above for cholesterol, but utilising enzymatic detectionof TG, such as by those methods described herein above. Such methodsinclude:

(i) U.S. Pat. No. 6,811,994 describes using selective surfactants andpolyethylene glycol modified enzymes to block lipoproteins other thanthe particular lipoprotein.(ii) WO2004/087945 and US2009/0226944 describe using selectivesurfactants to remove TG from nonLDL in a reaction not producingdetectable products then converting TG associated with LDL intodetectable products.(iii) WO2000/06112 describes using selective surfactants to remove TGfrom lipoprotein other than VLDL and/or IDL in a reaction not producingdetectable products then converting TG associated with VLDL and/or IDLinto detectable products.

Any of these methods, either individually or in combination may beapplied to the measurement of TG associated with specific lipoproteinsin the method of the present invention.

The detection method used in the assay methods of the present inventionis typically photometric, and the indirect product is generally chosensuch that it is detectable photometrically (e.g. by its absorbance,fluorescence or chemiluminescence at one or more pre-identifiedwavelengths).

A highly preferred method for signal generation is via hydrogenperoxide, which serves as a substrate for the enzymatic oxidation of acolour-producing substance. The oxidizable color producing reagent orreagents that react with formed hydrogen peroxide to produce thedetectable chemical product may be any molecule known in the art, theoxidized product of which can be measured by ultraviolet, visual, orinfra-red spectroscopy, or by fluorescence or luminescence.

Examples of suitable chromogenic reagents are Trinder reagents, which inthe presence of H₂O₂ react with a coupler to fin in colored products.Preferred examples of couplers are 4-aminoantipyrin (4AA),3-methyl-2-benzolinone hydrazone (MBTH),N-methyl-N-phenyl-4-aminoaniline (NCP-04),N-methyl-N-(3-methylphenyl)-4-aminoaniline (NCP-05),N-methyl-N-(3-methoxyphenyl)-4-aminoaniline (NCP-06),N-methyl-N-(3-methoxyphenyl)-3-methoxy-4-aminoaniline (NCP-07).Preferred examples of Trinder reagents are those forming products thatcan be colorimetrically determined at wavelengths at or above 600 nm:N-Ethyl-N-(2-hydroxy-3-sulfopropyl)-3,5-dimethyloxyaniline (DAOS),N-Ethyl-N-(2-hydroxy-3-sulfopropyl)-3,5-dimethoxy-4-fluoroaniline(FDAOS), N-(2-hydroxy-3-sulfopropyl)-3,5-dimethoxyaniline (HDAOS),N,N-bis(4-sulfobutyl)-3,5-dimethylaniline (MADS),N-ethyl-N-(2-hydroxy-3-sulfoprpyl)-3,5-dimethylaniline MAOS). Preferredexamples that are not Trinder reagents are:3,3′,5,5′-Tctramethylbenzidine (TMB),N-(3-sulfopropyl)-3,3′,5,5′-tetramethylbenzidine (TMBZ-PS),N,N-bis(2-hydroxy-3-sulfopropyl)tolidine (SAT Blue),N-(carboxymethyl-aminocarbonyl)-4,4-bis(dimethylamino)-biphenyl amine(DA64),10-(carboxymethylaminocarbonyl)-3,7-bis(dimethylamino)phenotiazine(DA67). The concentration of the chromogen is preferably 0.01-10 g/L,and is limited by solubility.

Examples of suitable fluorescent substrates are dihydrocalceins,dihydroethidium, dihydrofluoresceins, dihydrophenoxazine (Amplex red;10-acetyl-3,7-dihydroxyphenoxazine), and dihydrorhodamines.

Examples of suitable chemiluminescent substrates are luminol(3-aminotriphenylene complexes), Lumigen PS-2 and Lumigen PS-atto.

Non-ionic, anionic, cationic, and zwitterionic surfactants may be usedfor the purpose of cell lysis prior to measurement of colored indirectanalyte. Any suitable surfactant may be used that does not affect themeasurement, typically at concentrations between 0.01-10%, butpreferably between 0.1-2%.

Examples of suitable surfactants are the anionic surfactants amineaklylbenzene sulfonate (Ninate 411), sodium dioctylsulfosuccinate(Aerosol OT), sodium N-oleyl-N-methyltaurate (Geropon T-77), sodiumolefin sulfonate (Bioterge AS-40), sodium polyoxyethylene laurylsulphate (Standapol ES-1), and the non-ionic surfactants polyoxyethylenealkyl aryl ether (Triton X100 and Xl 14) and polyoxyethylene laurylalcohol (Chemal LA-9).

Where cell lysis has occurred, there will evidently be a correspondingrelease of haemoglobin and other cell products into the reaction medium.In order to gain maximum advantage from the lysis and reduce thebackground signal as far as possible, it is thus preferred in such casesto select a product (secondary analyte) for which detection is notinhibited by these released products. Secondary analytes detectable atwavelengths above 450 nm, preferably above 500 nm and most preferably600 nm or higher (e.g. 500 to 1400 nm or 600 to 1200 nm) are thuspreferred where cell lysis is used.

Physical separation of blood cells in the sample may be performed by anysuitable method including flow past a specific binder for the wholecells, but will most commonly be by filtration. It is notable thatfiltration to remove cells following reaction step (i) has considerableadvantages over filtration of the original blood sample. In particular,the reaction step can be carried out with a small volume of blood samplefrom a finger-stick. After one or more reagents have been added, thevolume of the reaction mixture is much greater and separation of thecells can be conducted without the dead-volume being significant. Thecells are also more dilute, whereby the cells upon filtration willspread more evenly over the filter surface and the filtration becomesmore effective.

Filtration as described herein is thus a preferred technique for use inset ii) of the methods of the present invention and correspondingfilters etc are suitable for supply with the kits of the invention. Somepreferred embodiments of such filtration are provided below, any ofwhich may be applied individually or in any combination to the methodsand kits of the invention.

In filtering, the sample is passed through a porous interface (filter,membrane or sieve) of small pore size, in which the cells are trapped.The porous interface may be solid (e.g. sintered glass) or fibrous (e.g.made of cellulose or glass), and the flow may be transversely through orlaterally. The sample may be passed through the filter by gravity,centrifugation, capillary forces, pressure or suction. The filteringmaterial may additionally contain reagents (e.g. lectins, antibodies)that capture the blood cells. Other methods use electrostaticattractions.

Suitable materials for transverse filtering are given in Table 2.Suitable materials for lateral flow filtering are Hemasep® L (Pall Corp)and LF1 and MF1 (Whatman).

TABLE 2 Filtration recovery of lipids and colored product TG CH HDLFilter type (%) (%) (%) No filtration 100 100 100 PAD 901 99 95 97 PADWhatman 98 92 94 QA Pall 102 96 97 Whatman grade 103 99 97 CF/CM30Gelman ITLC SG 103 96 99 Gelman A/E 103 98 98 Munktell filter 101 97 98paper

Mathematical treatment of enzyme progress curves based on absorbancereadings in order to calculate enzyme activities is well-known. Suchmathematical procedures, algorithms are based on the integratedMichelis-Menten equation (London J W (1977) Analytical Chemistry48:1716-9). However, the purpose of such algorithms is to determineenzyme activities by extrapolating the rate to zero time, they are notapplicable to the determination of product end-points.

Recently, one group reported that using a combination of fixed time andkinetic measurements allowed the reading of the progress curve to beterminated when it had reached 90-95% of the predicted end-point, ratherthan following it to the end. This allowed for a substantially shortenedassay time (Kvam (2009) Point of Care 8:16-20).

What the inventors have now surprisingly found is that in reactionswithout a measureable end-point it is still possible to estimate afictive end-point which is a direct representative of the true,non-measurable end-point (FIG. 1). This will allow for circumventing theproblems with fix-point measurements and their requirements forcalibrators and/or dry reagents. What is needed is a suitable algorithmand at least 50% of the progress curve (e.g. at least 60% or at least70%).

The algorithm used to predict the unmeasurable or fictive end-point inthe various aspects of the present invention can be any mathematicalequation that adequately describes the reaction progress, and might beused to describe the curve formed when such progress is plotted. Manysuch equations will be known to those of skill in the art. Usually thealgorithm will be one based upon a first order rate equation (e.g.Y=Y₀*e{circumflex over ( )}(−kX)) or a logistic function (e.g.Y=1/(1+e{circumflex over ( )}(−X))) (Table 3). Which algorithm will bemost preferable for a particular situation will depend on many factorssuch as how the measurement is performed (transmission or absorbance),reaction conditions (e.g. reaction temperature), as well as measurementinterval and other factors. It will, however, be a routine matter to fitthe curves from each suitable equation to the experimental data andthereby determine the closest fit in each case. Different instrumentplatforms may thus favour different curve fitting algorithms but asuitable algorithm in each case will be rapidly established. In manycases more than one algorithm will give acceptable results.

TABLE 3 Some Examples of reaction-curve fitting algorithms Type Equation1st order Y = Y_(max) + (Y₀ − Y_(max)) * e^(∧)(−kX) (Exponentialassociation/ dissociation) 1st order - 2 Y = Y_(max) − Y_(max) *e^(∧)(−k(X − X₀)) 1st order - 3 Y = Y_(max) + (Y₀ − Y_(max)) *e^(∧)(−k(X) + X * C Logistic - 1 Y = Y₀ + ( Y_(max) − Y₀)/(1 +10^(∧)(logX₅₀ − logX) * B (Sigmoidal) Logistic - 2 Y = Y_(max) + (Y₀ −Y_(max))/(1 + (X/X₅₀)^(B) (4 parameter logistic) Logistic - 3 Y = Y₀ +(Y_(max) − Y₀)/(1 + (X/X₅₀)^(B) Logistic -4 Y = Y_(max) + (Y₀ −Y_(max))/(1 + (X/X₅₀)^(B) + X * C Y0, Y at X = 0; Ymax, Y at X =infinite; k, rate constant; X50, X at half maximal Y; B, slope factor,steepness of slope; C, slope factor of parallel reaction..

When estimating an unmeasurable end-point, the measurement intervalshould preferably be chosen such that a minimum of parallel reactionsare occurring. However, it is not a strict requirement that parallelreactions cannot be present. The influence of such parallel reactionsmay be corrected for in a post-measurement analysis. This isparticularly true where at least two analytes are measured in the sampleand a known (absolute or approximate) relationship is present between atleast two of said analytes.

For instance, in the HDL assay the estimated unmeasurable end-point maybe influenced by a concomitant conversion of nonHDL, also when nonHDL,has been rendered temporarily unreactive by blocking reagents dependingon different factors such as (i) the measurement interval, (ii) reactiontemperature, (iii) HDL/nonHDL ratio. In a lipid-panel assay this may becorrected for in an iterative process involving HDL the simultaneouslymeasured total cholesterol and a calibration curve for nonHDL,determined in advance and placed in the information (e.g. barcode)supplied for the reagent, in the same way as done with the HDLcalibration curve. An example of how a post-analysis correction may becarried out is described in Example 6.

It would also be possible to take into account parallel reactionsoccurring in the algorithm used to estimate the unmeasurable end-point.For instance the reaction conditions may be formulated such thatparallel reactions are essentially linear during the measurementinterval, and can be represented in the algorithm by a linear term(Table 3).

One of the considerable advantages of the present assay method is theresultant decrease in the assay time. It is thus preferable that thetime from adding the reagent in step i) to end of the monitoring ofdetectable reaction products in step v) is no more than 10 minutes, morepreferably no greater than 8 minutes, and most preferably in the range 5to 7 minutes.

It is a further advantage of the present invention that it may becarried out on a minimal volume of blood. It is preferred, thereforethat the volume of whole blood sample in step i) of the assay method isno greater than 40 μL more preferably no greater than 20 μL and mostpreferably in the range of 10 to 15 μL.

The present inventors have additionally established that furtherreaction time may be saved by the early contact of the sample with the“blocking reagent” (the reagent which serves to temporarily preventreaction of said second component). This blocking reagent (many of whichare described herein) typically takes some time (several minutes) toreact with the second component. Thus, a typical point-of-care assay inwhich this blocking reagent was added at or around step iv) of themethod would then be delayed by several minutes to allow reaction ofthis blocking reagent.

The present inventors have now established that it is possible to add asuitable blocking reagent to the cell-containing blood sample (e.g.whole blood sample) without significantly lysing the cells such thatthis reagent can be reacting with the second component of the sampleduring the cell separation steps(s). Evidently, this reduces the need towait for the blocking reaction and can reduce the assay time by around30 seconds, preferably by around 1 minute. In this embodiment, theblocking reagent will typically be included in the dilution mixtureadded at step i), or may alternatively be added after dilution, eitherbefore cell separation or after that cell separation process. In anycase, if the blocking reagent can be added earlier than previously thenassay time can be saved.

It is important in multi-analyte (panel) assays that where the blockingreagent is added before the sample is divided into portions for specificreaction, the blocking reagent should not interfere with detection ofthe various analytes. Where then analytes are HDL-CH, total-CH andtotal-TG, for example, the blocking reagent may be added at any of stepsi) to iii) without interference because the reagent causes blocking ofnon-HDL-CH (the second component) and thus allows specific reaction ofHDL-CH. With regard to the total-CH and total-TG, although the blockingreagent will be present, the reagent mixture for specific reaction ofthese need only contain a suitable surfactant to release the total CH ortotal TG in spite of the block. Such surfactants will be known andeasily selected by the skilled worker and so no interference withtotal-CH or total TG need take place.

In the kits of the present invention, the first reagent mixture ispreferably formulated together with the first reagent mixture of the HDLreaction as a single reagent mixture. This reduces the number of steps,and thus reduces the assay time, and furthermore reduces the reagentstorage and handling demands on the assay equipment, allowing the methodto be practiced with less sophisticated automatic analysers.

The kit may additionally contain a lysing agent to cause cell lysisallowing determining the hematocrit.

The invention will now be further illustrated by the followingnon-limiting examples, and the attached. Figures, in which:

FIG. 1 demonstrates that the estimated, non-measurable end point of HDLassociated cholesterol in the presence of cholesterol associated withnonHDL coincides with the measurable end-point of the same amount of HDLassociated cholesterol in the absence of cholesterol associated withnonHDL.

FIG. 2 compares end-point, fix-point and estimated end-pointmeasurements in the Roche TG assay and shows that whereas fix-pointmeasurements are strongly dependent upon reagent storage time, end-pointand estimated end-point measurements are not.

FIG. 3. demonstrates the usefulness of end-point estimation in themeasurement of HDL using a commercial HDL reagent. Estimated end-pointscorrelated excellently with the determined HDL-levels.

FIG. 4 compares the HDL levels obtained by the estimated end-pointmethod in an Afinion point-of-care instrument with those obtained by aclinical laboratory method. The figure gives a comparison of HDL levelsdetermined by Afinion method and laboratory method using each of the“1st order” and the “Logistic” fitting algorithms.

FIG. 5 compares the HDL levels obtained by the estimated end-pointmethod in an Afinion point-of-care instrument with those obtained by acommercial point-of-care method. The figure shows a comparison of HDLlevels determined by the Afinion method and a comparative commercialpoint of care method. The fit is noticeably poorer than for embodimentsof the current invention (FIG. 4).

FIG. 6 demonstrates that the nonHDL blocking buffer, needed in the HDLassay, may be used as a general dilution buffer. The TG levels obtainedin an Afinion point-of-care instrument, using blocking buffer as adilution buffer, is compared with those obtained by a clinicallaboratory method.

FIG. 7 compares the nonHDL levels obtained by the estimated end-pointmethod in an Afinion point-of-care instrument with those obtained by aCRMLN certified laboratory.

FIG. 8 compares whole blood and plasma lipid profiles determined on anAfinion point-of-care analyzer.

EXAMPLES Example 1

Estimation of a Fictive End Measurement of HDL Associated Cholesterol inthe Presence of Cholesterol Associated with Non-HDL.

A sample containing 66 mg/dL HDL, 255 mg/dL LDL and 300 mg/dL TG wasanalyzed on a Cobas Mira plus instrument (ABX Diagnostics) using WakoHDL-C L-type reagents and protocol (Table 4). The absorbance wasmonitored at 600 nm (FIG. 1, closed dots). A calibrator containing 64mg/dL HDL but no significant amount of LDL was also analyzed in the sameway (FIG. 1, open dots). The progress curves for the samples were closeto identical for the first 200-300 s, but then diverged as thecholesterol of the non-HDL in the sample became unblocked and was beingconverted. The conversion of HDL associated cholesterol in the presenceof cholesterol associated with other nonHDL thus has no measurableend-point. The only measurable end point will be that of the cholesterolpresent in HDL+nonHDL (i.e. total cholesterol). However, from the first200 s (thick line) of the progress curve of the sample, a fictiveend-point for the HDL associated cholesterol could be computed (dottedline), using the first-order algorithm

Y=Y _(max)(1−e ^((−K(X−X0)))),

which coincided with the true end-point of a calibrator containing onlyHDL of almost the same concentration, 64 mg/dL versus 66 mg/dL. Otherwell known algorithms for following the kinetics of enzymatic reactionscould be used in an analogous way.

TABLE 4 Wako HDL-C L-type reagent and protocol. R1 Pretreatment 5minutes 37° C. R2 Incubation 5 minutes 37° C. R1 pretreatment Good'sbuffer, pH 7.0    30 mmol/L 4-aminoantipyrine   0.9 mmol/L peroxidase  2400 U/L ascorbate oxidase   2700 U/L anti-human β-lipoproteinantibody R2 enzyme reagent Good's buffer, pH 7.0    30 mmol/Lcholesterol esterase   4000 U/L cholesterol oxidase  20000 U/L F-DAOS  0.8 mmol/L

Example 2

Comparison of End-Point, Fix-Point and Estimated End-Point Measurementsin the TG Assay.

Roche TG reagent (Table 5) was stored at 25° C. and samples were takenout at different time points and used to measure a plasma samplecontaining 186 mg/dL triglyceride. The plasma sample was stored inaliquots frozen at −40° C. and thawed prior to measurement. Themeasurements were performed on a LabSystems Multiscan RC plate reader atambient temperature (20-22° C.). The progress curves were followed untilreaction completion and the end-points determined. In addition, theend-points were estimated based upon the first 300 s of the progresscurves using a 4 parameter logistic function. The obtained estimatedend-points were compared to true end-points and to the absorbance at theend of the time interval used for the estimation, 300 s. As seen in FIG.2 estimated end-points followed true end-points whereas fix-pointmeasurements were already falling off dramatically in comparison withthe true end-points after as little as 5 months storage. Thus, whileboth end-point estimation and fixed-point methods allow a test period ofonly 300 seconds, fix-point measurements would need inclusion ofcalibrators to give correct values for the samples over any reasonablelife span of the reagent. In contrast, estimated end-point computationswould maintain accuracy without included calibrators.

TABLE 5 Roche triglyceride GPO-PAP reagent buffer Pipes 50 mmol/L, pH6.8, containing 40 mmol/L Mg2+, sodium cholate 0.20 mmol/L, and 1 μmol/Lpotassium hexacyanoferrate (II) ATP  1.4 mmol/L 4-aminoantipyrine  0.13mmol/L 4-chlorophenol  4.7 mmol/L fatty alcohol 0.65% polyglycol etherlipoprotein lipase  5000 U/L (from Pseudomonas spec.) glycerokinase  190 U/L (from Bacillus stearothermoohilus) glycerol phosphate  2500U/L (from E. coli) oxidase peroxidase   100 U/L (from horseradish)

Example 3

End-Point Estimation of the Roche HDL Assay

The HDL level of 8 different serum samples was determined on a ‘CobasMira plus’ instrument from ABX technologies using HDLC3 reagent fromRoche (Table 6). Using the initial 100 seconds of the progress curves,fictive end-points were estimated using a 4-parameter logistic function.As shown in FIG. 3 the estimated end-points (in absorbance units)correlated excellently with the determined HDL levels.

TABLE 6 Roche Direct HDLC3 reagents. R1 Pretreatment 5 minutes 37° C. R2Incubation 5 minutes 37° C. R1 pretreatment Hepes, pH 7.4   10 mmol/LCHES, pH 7.4   97 mmol/L dextran sulfate  1.5 g/L magnesiumnitratehexahydrate   12 mmol/L HSDA   1 mmol/L Ascorbate oxidase 16.7μkat/L (Eupcnicillium sp) R2 enzyme reagent Hepes, pH 7.0   10 mmol/LPEG-cholesterol esterase 3.33 μkact/L (Pseudomonas spec.)PEG-cholesterol oxidase  127 μkat/L (Streptomyces spec.) peroxidase  333μkat/L (horseradish) 4-aminoantipyrine  2.5 mmol/L

Example 4 Example 4

The Afinion HDL Assay (Point-of-Care)—Comparison to Large ClinicalInstrument Assay Method.

The HDL levels of 49 scrum samples were determined on an Adviainstrument and reagents from Siemens, the reference method. The samplesalso were measured by the estimated end-point method using Wako HDL-CL-type reagents (Table 4) on an Afinion point-of-care apparatus.Progress curves were monitored for 160 seconds and end-points estimatedusing two different algorithms: a 1^(st) order equation (Table 3,type 1) and a logistic equation (Table 3, type log-transformed). Theestimated end-points were converted to concentrations using acalibration curves established on Afinion in a separate experiment usingcalibrators from Trina Bionostics, whose HDL levels had been determinedon Cobas Mira plus using HDL direct CP reagents from ABX Pentra (Table7). FIG. 4 shows for the 44 serum samples the correlation of the Afinionmethod with the reference method (1st order prediction—closed circles,logistic fitting, open circles). The mean HDL level as measured by theAfinion method was 0.4% (0.4 mg/dL) and 1.1% (0.6 mg/dL) lower comparedto the reference method and the slope factor was 0.96 and 1.06 using1^(st) order and logistic curve fitting, respectively.

TABLE 7 ABX HDL Direct CP reagents. R1 Pretreatment 5 minutes 37° C. R2Incubation 5 minutes 37° C. R1 pretreatment Good's buffer, pH 7.0cholesterol oxidase 1000 U/L peroxidasc 1300 ppg U/L DSBmt   1 mmol/Laccelerator   1 mmol/L R2 enzyme reagent Good's buffer, pH 7.0cholesterol esterase 1500 U/L 4-aminoantipyrine   1 mmol/L ascorbic acidoxidase 3000 U/L detergent   2% restrainer 0.15%

Example 5

The Afinion HDL Assay—Comparison to Point of Care Assay Method

The HDL level of 42 serum samples were determined on a point-of-careassay system from Cholestech, LDX system Lipid profile Glu. The samplesalso were measured and HDL values computed by the Afinion HDL method,essentially as described in Example 4. FIG. 5 shows for the 42 serumsamples the correlation of the Afinion and LDX methods. As the LDXmethod reports HDL values below 15 mg/dL as <15 mg/dL and values above100 mg/dL as >100 mg/dL, these values (open circles) were omitted in thelinear regression (n=10). The mean HDL level as measured by the Afinionwas 1% (0.5 mg/dL) higher compared to the LDX method and the slopefactor was 1.03.

Example 6

Correcting the Estimated End-Point of HDL in a Multi-Component AssayMethod for Influence of Parallel Conversion of nonHDL.

To exemplify the possibility of correcting the HDL assay for aninfluence of simultaneously converted nonHDL by postanalysiscomputations based upon concomitant measurement of total CH in thesample, the following experiment was performed:

(i) a calibration curve for HDL was constructed from a preparation ofpure HDL (Wako High Unit HDL-C) using a Cobas Mira plus instrument andWako HDL-C L-type reagents and protocol (Table 4). Estimated end-pointabsorbance from the first 300 s of the progress curves and curve fittingusing a 1^(St) order function were used in the construction of thecalibration curve:

HDL=1.2+136.3ABS  (1)

(ii) a calibration curve for the influence on the measured HDL value bya parallel conversion of nonHDL was constructed from mixtures of pureHDL and pure LDL (Wako High Unit LDL-C), two different concentrations ofHDL and 4 different concentrations of LDL. The increase in the HDLmeasured was plotted as a function of the nonHDL concentration. Theincrease was exponentially correlated to the nonHDL concentration

HDL increase=2.433 exp{circumflex over ( )}0.008499nonHDL  (2)

The concentration of nonHDL was calculated from the difference betweenCH (total cholesterol measured) and HDL:

nonHDL=CH−HDL  (3)

A serum sample containing 111 mg/dL HDL was mixed at two differentconcentrations with different amounts of pure LDL. The CH and HDL levelswere measured on a Cobas Mira plus instrument with ABX PentraCholesterol CP and the Wako HDL-C L-type reagents and protocol,respectively.

Measured values were on average 48% overestimated. After iterativecorrection using the equations (1) to (3) above the mean overestimationwas decreased to 5% (Table 8).

TABLE 8 Influence of nonHDL on HDL measurement and correction thereof.HDL HDL HDL Sample CH true measured postanalysis corr 36-1 117 36 41 3636-2 231 36 52 40 36-3 336 36 68 38 62-1 204 62 70 62 62-2 314 62 84 6462-3 428 62 120 69 Grand mean 49 72.5 51.5

Samples were run in 2-3 replicates.

Example 7—Elimination of Cell Lysis by Blocking Reagent

2 μL of whole blood was added to 4000_, of HDL-C L-type R1 (Wako) added0, 60, 90 or 120 mmol/L of NaCl. After mixing the absorbance of thesamples were measured at 660 nm. At this wavelength there is noabsorption from hemoglobin but strong interference (through scatteringof light) from intact cells. While R1 added no NaCl caused almostcomplete lysis, R1 added 120 mmol/L NaCl caused insignificant celllysis.

NaCl added Absorption at 660 nm Hemolysis (mmol/L) (AU) (%) 0 0.062 10060 1.306 23 90 1.569 7 120 1.615 4

As can be seen, reduction of cell lysis to below 5% is achievable byappropriate choice of ionic strength in reagent R1.

Example 8 Use of Blocking Reagent Comprised within Non-Lytic DilutionBuffer

Effect of Using HDL Reagent R1 as General Dilution Buffer on Measurementof Triglycerides.

The TO levels of 46 serum samples were determined on Afinionpoint-of-care instrument using HDL reagent R1 (Table) as the generaldilution buffer and Roche TO reagent (Table 5).

15 μL sample was diluted into 280 μL of HDL reagent R1 and 58 μL wastransferred to 100 μL of TG reagent. Reaction end-points were determinedand converted to concentrations using a calibration curve established onAfinion in a separate experiment using calibrators that had beenpreviously quantified at a CRMLN laboratory (Seattle, USA). In aparallel experiment the same samples were determined in a separatelaboratory (Fürst Medical Laboratory, Oslo, Norway) using an Adviainstrument and Siemens reagents. The samples compared very well betweenthe two methods and there was no indication of a significantinterference from the HDL R1 dilution buffer on the Afinion TG results(FIG. 6).

Table Dilution buffer Dilution buffer Good's buffer, pH 7.0 NaCl   175mmol/L 4-aminoantipyrine  0.9 mmol/L peroxidase  2400 U/L Ascorbateoxidase  2700 U/L Anti-human apo-β-lipoprotein antibody

Example 9—Dilution of Whole Blood in Isotonic HDL R1 and Filtration toObtain an Essentially Cell Free Filtrate

The applicability of HDL R1 (non-HDL blocking) reagent as a generaldilution and filtration buffer for whole blood was investigated in anAfinion instrument supplied with filtration capability by placing afilter pad at the bottom of well nr.2. The dilution/filtration bufferwas HDL-C L-type reagent 1 (R1) (Table 4) made nonlytic by adding NaClto 175 mM.

Whole blood was drawn into a 154 sample device by capillary forces andinserted into the Multiwell cartridge which was placed in theinstrument. The sample was emptied and mixed into 250 μL of the modifiedHDL R1, and the entire volume transferred into the well containing thefilter pad. The membrane tube of the Multiwell cartridge was thenpositioned tightly over the filter pad. When below ambient pressure wasapplied to the open end of the membrane tube the diluted whole bloodflowed into the filter pad and whereas blood cells were trapped in thefilter, the diluted plasma flowed through the filter and into themembrane tube. When pressure started rising (sucking air) the membranetube was removed from the filter, lowered into an empty well and forcedto release the filtered plasma by applying an above ambient pressure.Blood contamination was measured after converting contaminatinghaemoglobin to methhemoglobin using NaNO₂ and measuring the absorbanceat a wavelength of 410 nm. The absorbance was converted to % Hb byinterpolation from a calibration curve constructed from knownconcentrations of methhemoglobin. Results given in Table 9 are thecompounded results of hemolysis caused by dilution into the R1 reagent,hemolysis caused by filtration, and contamination by blood cells, nottrapped in the filter pad.

TABLE 9 Total haemoglobin contamination after dilution into R1 andfiltration. Whole blood % Filtrate % Hb Filtertype hematocritcontamination Whatman 37 0.9 Whatman 61 1.2 Millipore AP25 37 0.5Millipore AP25 61 1.5 Millipore 2 37 0.8 Millipore 2 61 1.2 Mean of 5-6replicates

Example 10

Estimation of a Fictive End Measurement of nonHDL Associated Cholesterolin the Presence of Cholesterol Associated with HDL. Comparison toCertified Laboratory Method.

A method for the direct determination of nonHDL was constructed by usinga HDL specific block polymer that protects HDL while cholesterolassociated with all other lipoproteins (nonHDL) are converted to adetectable product (HDL-X; Wako, Japan). As source for the HDL specificblock polymer was used Reagent 1 from the Wako HDL-C L-M/2-PM reagent(Wako, Japan). The exact composition of Reagent 1 is not available, butit is disclosed to contain Good's buffer pH 7.0, cholesterol esterase,cholesterol oxidase, HMMPS, catalase, ascorbate oxidase and HDLblock-polymer. By supplementing Reagent 1 with peroxidise (6000 U/L),4-aminoantipurin (0.8 mmol/L) and sodium azide (0.025%) a reagent wasformulated that converted nonHDL into a detectable product while HDL wastemporarily protected.

2 μL of calibrator or plasma sample was mixed with 1500_, ofsupplemented Reagent 1. The reaction was performed at 37 C and monitoredat 600 nm using a Cobas Mira plus instrument. End-point absorbance wasestimated from the first 300 s of the progress curve using a 4 parameterlogistic function (Table 3).

Calibrators and samples had been previously quantified at a CRMLNlaboratory (Seattle, USA) with respect to CH, HDL and TG. From thesevalues nonHDL values were calculated according to

nonHDL=CH−HDL.

The nonHDL values of the 9 serum samples determined by the method of theinvention correlated very well with the nonHDL values computed from theCRMLN values (FIG. 7)

Example 11

Whole Blood and Plasma Lipid Profiles Determined on an AfinionPoint-of-Care Analyzer.

The plasma levels of CH, TG and HDL were determined on 15 μL of freshwhole blood obtained from 60 healthy volunteers, using an Afinionanalyzer. From these data the LDL levels were computed in mg/dL usingthe Friedewald equation, LDL=CH−(HDL+TG/5). The plasma fraction of thesame samples were also obtained (by centrifuging at 1000 g for 10minutes) and measured on the Minion for the same analytes. The reagentsused were those described in Tables 4 (HDL), 5 (TG) and 10 (CH).Reaction end-points were determined for TG and CH. The unmeasurableend-point of the HDL reaction was estimated using the first 100 s of theprogress curve and curve fitting using a 1^(st) order function (Table3). Whole blood hematocrit was determined as described in Example 9 andwhole blood results were converted to plasma results by dividing eachresult with (1-hematocrit) using the hematocrit value obtained for eachspecific whole blood sample. FIG. 8 depicts for the 60 samples thecorrelation between the plasma levels obtained in whole blood andplasma.

TABLE 10 Roche Chol2 reagent buffer Pipes 225 mmol/L, pH 6.8, containing10 mmol/L Mg2+ sodium cholate  0.6 mmol/L 4-aminoantipyrine  0.45 mmol/Lphenol  12.6 mmol/L fatty alchol polyglycol ether 3% cholesterolesterase  1500 U/L (Pseudomonas spec.) cholesterol oxidase   450 U/L(E.coli) peroxidase   750 U/L (horseradish)

1-33. (canceled)
 34. A method for correcting a high density lipoprotein(HDL) assay, said method comprising: measuring a total cholesterol (CH)value for a sample; measuring a measured high density lipoprotein (HDL)value for the sample; calculating a non-HDL value by subtracting themeasured HDL value from the CH value; calculating a corrected HDL valueusing a function relating the non-HDL value and an increase in HDL,wherein said increase in HDL is calculated by subtracting the correctedHDL value from the measured HDL value and said corrected HDL value iscalculated by subtracting said increase in HDL from said measured HDLvalue.
 35. The method of claim 34, wherein the HDL value is provided byestimating an unmeasurable or fictive endpoint of an HDL assay performedon the sample.
 36. The method of claim 34, further comprising: providinga calibration sample comprising a known amount of HDL and a known amountof non-HDL; providing a measured HDL value for the calibration sample;and calculating an increase in measured HDL for the calibration sampleby subtracting the known about of HDL in the calibration sample from themeasured HDL value for the calibration sample.
 37. The method of claim36, further comprising producing a calibration curve comprising saidincrease in measured HDL for the calibration sample plotted as afunction of the amount of non-HDL.
 38. The method of claim 36, whereinsaid function relates said increase in measured HDL for the calibrationsample to the amount of non-HDL.
 39. The method of claim 34, whereinsaid function is an exponential function.
 40. The method of claim 34,wherein said function is provided to a user with an HDL assay reagent.41. The method of claim 34, wherein said function is provided as abarcode.
 42. The method of claim 34, wherein said sample is a bloodsample.
 43. The method of claim 34, wherein said measured HDL value ismeasured using spectrophotometry.
 44. The method of claim 34, whereinsaid measured HDL value is determined by estimating an end-pointabsorbance by fitting a curve to a portion of a reaction progress curve.45. The method of claim 44, wherein said portion of said reactionprogress curve is the first 300 seconds of the progress curve.
 46. Themethod of claim 44, wherein said fitting fits a curve describes by a 1storder function to said reaction progress curve.
 47. A method forcorrecting a high density lipoprotein (HDL) assay, said methodcomprising: measuring a total cholesterol (CH) value for a sample;measuring a measured high density lipoprotein (HDL) value for the sampleusing a first equation relating HDL to absorbance; calculating a non-HDLvalue using a second equation relating the non-DHL value to the CH valueand the measured HDL value; calculating an increase in HDL using a thirdequation relating the increase in HDL and the non-HDL value; andproviding a corrected HDL value by performing an iterative correction ofsaid measured HDL value using the first equation, second equation, andthird equation.
 48. The method of claim 47 wherein said second equationis:non-DHL value=CH value−measured HDL value.
 49. The method of claim 47,wherein said third equation comprises an exponential function.
 50. Themethod of claim 14, wherein said first equation comprises a linearfunction.
 51. The method of claim 47, wherein said first equation isprovided by estimating an end-point absorbance by fitting a curve to aportion of a reaction progress curve.
 52. The method of claim 47,wherein said third equation is provided by: providing a calibrationsample comprising a known amount of HDL and a known amount of non-HDL;providing a measured HDL value for the calibration sample; andcalculating an increase in measured HDL for the calibration sample bysubtracting the known about of HDL in the calibration sample from themeasured HDL value for the calibration sample; and identifying afunction describing a curve relating said increase in measured HDL forthe calibration sample to the amount of non-HDL.